The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

) which contains the Upper Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest European chalk ' province'. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment. Chalk is a limestone that consists of

coccolith Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as ''Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere''. ...

biomicrite. A biomicrite is a limestone composed of fossil debris ("bio") and calcium carbonate mud (" micrite"). Most of the fossil debris in chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as coccolithophores. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of foraminifera, ostracods and

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

. The coccolithophores lived in the upper part of the water column. When they died, the microscopic calcium carbonate plates, which formed their shells settled downward through the ocean water and accumulated on the ocean bottom to form a thick layer of calcareous

ooze Ooze may refer to: * Pelagic sediments, fine-grained sediments on the ocean floor, containing at least 30% biogenous material Games * Ooze (''Dungeons & Dragons''), a type of monster in the ''Dungeons & Dragons'' role-playing game * ''The Ooze'' ...

, which eventually became the Chalk Group. The Chalk Group usually shows few signs of bedding, other than lines of flint nodules which become common in the upper part.

Nodules Nodule may refer to: *Nodule (geology), a small rock or mineral cluster *Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, p ...

of the mineral pyrite also occur and are usually oxidized to brown

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...

on exposed surfaces. Well-known outcrops include the White Cliffs of Dover, Beachy Head, the southern coastal cliffs of the Isle of Wight and the quarries and motorway cuttings at

Blue Bell Hill Blue Bell Hill is a chalk hill between Maidstone and Rochester in the English county of Kent. It overlooks the River Medway and is part of the North Downs. Settlements on the hill include the Walderslade suburb of Chatham and the villages of B ...

, Kent, (which has been classified as a ''

Site of Special Scientific Interest A Site of Special Scientific Interest (SSSI) in Great Britain or an Area of Special Scientific Interest (ASSI) in the Isle of Man and Northern Ireland is a conservation designation denoting a protected area in the United Kingdom and Isle of ...

'') and at the

Stokenchurch Gap Aston Rowant Cutting is a geological Site of Special Scientific Interest south of Aston Rowant in Oxfordshire. It is a Geological Conservation Review site. Aston Rowant Cutting, also known as the Stokenchurch Gap or Aston Hill cutting, or loca ...

on the

Oxfordshire Oxfordshire is a ceremonial and non-metropolitan county in the north west of South East England. It is a mainly rural county, with its largest settlement being the city of Oxford. The county is a centre of research and development, primarily ...

Buckinghamshire Buckinghamshire (), abbreviated Bucks, is a ceremonial county in South East England that borders Greater London to the south-east, Berkshire to the south, Oxfordshire to the west, Northamptonshire to the north, Bedfordshire to the north-ea ...

border where the

M40 motorway The M40 motorway links London, Oxford and Birmingham in England, a distance of approximately . The motorway is dual three lanes except for junction 1A to junction 3 (which is dual four lanes) a short section in-between the exit and entry slip-r ...

cuts through the

Aston Rowant National Nature Reserve Aston Rowant National Nature Reserve is located on the north-west escarpment of the Chiltern Hills, in the Chilterns Area of Outstanding Natural Beauty. It has an area of , and most of it is a biological Site of Special Scientific Interest. I ...

Subdivisions

The Chalk Group is now divided into a ''White Chalk Subgroup'' and a ''Grey Chalk Subgroup'', both of which are further subdivided into formations. These modern divisions replace numerous earlier divisions, references to which occur widely on geological maps and in other geological literature. Previously no subgroups were defined but three formations were identified; the Upper Chalk, Middle Chalk and Lower Chalk. Different formations are defined within the 'northern' and 'southern' provinces, from Norfolk northwards and south of the Thames valley respectively. A 'transitional province' between the two and covering much of

East Anglia East Anglia is an area in the East of England, often defined as including the counties of Norfolk, Suffolk and Cambridgeshire. The name derives from the Anglo-Saxon kingdom of the East Angles, a people whose name originated in Anglia, in ...

and the

Chiltern Hills The Chiltern Hills is a chalk escarpment in England. The area, northwest of London, covers stretching from Goring-on-Thames in the southwest to Hitchin in the northeast - across Oxfordshire, Buckinghamshire, Hertfordshire, and Bedfordshire. ...

is also recognised. A different approach again is taken as regards the succession beneath the North Sea.

Grey Chalk Subgroup

The Grey Chalk Subgroup (formerly the Lower Chalk minus the Plenus Marls) is usually relatively soft and greyish in colour. It is also the most fossiliferous (especially for

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

fossils). The

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

of this subgroup usually begin with the 'Glauconitic Marl Member' (formerly known as the Glauconitic or Chloritic Marl), named after the grains of the green minerals glauconite and

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

which it contains. The remainder of the subgroup is argillaceous in its lower part (the West Melbury Marly Chalk Formation (formerly the 'Chalk Marl') and becomes progressively purer in the 'Zig-zag Chalk Formation' (the former 'Grey Chalk'). In the central Chilterns the two parts are separated by the hard

Totternhoe Stone 250 px, The characteristic checkerboard design of many Bedfordshire churches built with alternating chalk and flint blocks: St Mary's (Luton). Totternhoe Stone is a relatively hard chalk outcropping in the middle of the Lower Chalk in the Chilt ...

, which forms a prominent scarp in some places. There are few, if any, flint nodules present. These two formations are not recognised within the northern province i.e. the outcrop north from

to Yorkshire, where the entire sequence is now referred to as the 'Ferriby Chalk Formation'. The thickness of the Grey Chalk Subgroup strata varies, averaging around , depending upon the location. They often contains fossils such as the ammonites ''

Schloenbachia ''Schloenbachia'' is a genus of ammonoid cephalopods from the Cenomanian stage of the Upper Cretaceous, and type for the Schloenbachiidae, a family within the ammonitid Hoplitoidea Hoplitoidea, formerly Hoplitaceae, is a s ...

'', '' Scaphites'', and ''

Mantelliceras ''Mantelliceras'' is an extinct ammonoid cephalopod genus belonging to the family Acanthoceratidae and type for the subfamily Mantelliceratinae, that lived from the Late Albian to the late Cenomanian stage of the Late Cretaceous. Description '' ...

'', the belemnite ''

Actinocamax ''Actinocamax'' (from el, ἀκτίς , 'ray' and el, κάμαξ 'stake') is a genus of belemnite, an extinct group of cephalopods. See also * Belemnite * List of belemnites This list of belemnite genera is an attempt to create a compreh ...

'', and the

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

'' Inoceramus'' and '' Ostrea''. Chalk & greensand at Lulworth

White Chalk Subgroup

The White Chalk Subgroup includes what were formerly designated the Middle Chalk and Upper Chalk Formations, together with the Plenus Marls (topmost part of the former Lower Chalk Formation). In the southern province it is divided in the following way (youngest/uppermost at top): * Portsdown Chalk Formation (formerly part of 'Upper Chalk' and the equivalent of Rowe Chalk Formation, below) * Culver Chalk Formation (formerly part of 'Upper Chalk') ** Spetisbury Chalk Member (formerly part of 'Upper Chalk') ** Tarrant Chalk Member (formerly part of 'Upper Chalk') * Newhaven Chalk Formation (formerly part of 'Upper Chalk') * Seaford Chalk Formation (formerly part of 'Upper Chalk') * Lewes Nodular Chalk Formation (formerly part of 'Upper Chalk') * New Pit Chalk Formation (formerly part of 'Middle Chalk') * Holywell Nodular Chalk Formation (formerly part of 'Middle Chalk') ** Plenus Marls Member In the northern province the sequence is divided thus: * Rowe Chalk Formation (formerly part of 'Upper Chalk' and the equivalent of Portsdown Chalk Formation, above) * Flamborough Chalk Formation (formerly part of 'Upper Chalk') * Burnham Chalk Formation (formerly part of 'Upper Chalk') * Welton Chalk Formation (formerly 'Middle Chalk') ** Plenus Marls Member In the southern province, the former Middle Chalk, now the Holywell Nodular Chalk Formation and overlying New Pit Formation, averages about in thickness. The sparse fossils found in this sequence include the brachiopod '' Terebratulina'' and the echinoid '' Conulus''. The former Upper Chalk by comparison is softer than the underlying sequence and the flint nodules it contains are far more abundant in the South of England, although in Yorkshire the underlying strata have the highest concentration of flints. It may contain ammonite and

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

fossils in some nodular layers. The thickness of this sequence varies greatly, often averaging around . Fossils may be abundant and include the

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

Spondylus ''Spondylus'' is a genus of bivalve molluscs, the only genus in the family (biology), family Spondylidae.MolluscaBase (2019). MolluscaBase. Spondylus Linnaeus, 1758. Accessed through: World Register of Marine Species at: http://www.marinespecies ...

'', the brachiopods ''Terebratulina'' and ''Gibbithyris'', the

echinoids Sea urchins () are spine (zoology), spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard s ...

''Sternotaxis'', ''

Micraster ''Micraster'' is an extinct genus of echinoids from the Late Cretaceous to the early Eocene. Its remains have been found in Africa, Antarctica, Europe, and North America. Micraster was an infaunal echinoid living in a burrow below the sediment s ...

'', ''

Echinocorys ''Echinocorys'' is an extinct genus of echinoids that lived from the Late Cretaceous to the Paleocene. The genus belongs to the Holasteridae family. Its remains have been found in Asia, Europe, Australia (Oceania) and North America. Sources * ' ...

'', and ''

Tylocidaris ''Tylocidaris'' is an extinct genus of echinoids that lived from the Early Cretaceous to the Eocene. Its remains have been found in Europe and North America. Sources * ''Fossils'' (Smithsonian Handbooks) by David Ward (Page 177) External l ...

'', the crinoid ''

Marsupites ''Marsupites'' is an extinct genus of crinoids from the Santonian stage of the Late Cretaceous.sponge '' Porosphaera''. A possible azhdarchoid pterosaur is known from

Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by t ...

-aged rocks that form part of the Upper Chalk, making it the youngest known pterosaur discovered to date in England. The youngest beds of the sequence are found on the coast of Norfolk. Other fossils commonly found in this formation include: solitary corals (such as '' Parasmilia''), marine worm tubes (such as ''

Rotularia ''Rotularia'' is an extinct genus of planispirally coiled fossil polychaete worms in the family Serpulidae. Owing to the gastropod-like shape of ''Rotularia'', many authors in the past interpreted this genus as being sea snails in the family ...

''), bryozoans, scattered fragments of

starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...

and fish remains (including shark teeth such as ''

Cretolamna ''Cretalamna'' is a genus of extinct otodontid shark that lived from the latest Early Cretaceous to Eocene epoch (about 103 to 46 million years ago). It is considered by many to be the ancestor of the largest sharks to have ever lived, ''Otodu ...

'' and ''

Squalicorax ''Squalicorax'', commonly known as the crow shark, is a genus of extinct lamniform shark known to have lived during the Cretaceous period. The genus had a global distribution in the Late Cretaceous epoch. Multiple species within this genus are c ...

'').

Chalk landscapes of England

The Chalk outcrops across large parts of southern and eastern England and forms a significant number of the major physiographical features. Whilst it has been postulated that a chalk cover was laid down across just about all of England and Wales during Cretaceous times, subsequent uplift and erosion has resulted in it remaining only southeast of a line drawn roughly between The Wash and Lyme Bay in Dorset and eastwards from the

scarp Scarp may refer to: Landforms and geology * Cliff, a significant vertical, or near vertical, rock exposure * Escarpment, a steep slope or long rock that occurs from erosion or faulting and separates two relatively level areas of differing elevatio ...

s of the Lincolnshire and Yorkshire Wolds. Gentle folding of the Mesozoic rocks of this region during the Alpine orogeny has produced the London Basin and the Weald–Artois Anticline, the Hampshire Basin and the less gentle Purbeck-Wight monocline. The broadly western margin of the Chalk outcrop is marked, from northeast to southwest, to south by the Chalk downlands of the Yorkshire Wolds, the Lincolnshire Wolds, a subdued feature through western Norfolk, including

Breckland Breckland in Norfolk and Suffolk is a 39,433 hectare Special Protection Area (SPA) under the European Union Directive on the Conservation of Wild Birds. The SPA partly overlaps the 7,544 hectare Breckland Special Area of Conservation. As a la ...

, the

, the Berkshire Downs,

Marlborough Downs The North Wessex Downs Area of Outstanding Natural Beauty (AONB) is located in the English counties of Berkshire, Hampshire, Oxfordshire and Wiltshire. The name ''North Wessex Downs'' is not a traditional one, the area covered being better kno ...

and the western margins of

Salisbury Plain Salisbury Plain is a chalk plateau in the south western part of central southern England covering . It is part of a system of chalk downlands throughout eastern and southern England formed by the rocks of the Chalk Group and largely lies wi ...

and

Cranborne Chase Cranborne Chase () is an area of central southern England, straddling the counties Dorset, Hampshire and Wiltshire. It is part of the Cranborne Chase and West Wiltshire Downs Area of Outstanding Natural Beauty (AONB). The area is dominated by, ...

and the North and South Dorset Downs. In parts of the Thames Basin and eastern East Anglia the Chalk is concealed by later deposits, as is the case too within the Hampshire Basin. Ivinghoe Beacon seen from The Ridgeway

Only where the Weald–Artois Anticline has been 'unroofed' by erosion i.e. within the Weald is the Chalk entirely absent. In this area the long north-facing scarp of the South Downs and the longer south-facing scarp of the North Downs face one another across the Weald. For similar reasons, the Chalk is largely absent from the rather smaller area to the south of the Purbeck-Wight Monocline, save for the downs immediately north of

Ventnor Ventnor () is a seaside resort and civil parish established in the Victorian era on the southeast coast of the Isle of Wight, England, from Newport. It is situated south of St Boniface Down, and built on steep slopes leading down to the sea. ...

on the Isle of Wight. Some of the best exposures of the Chalk are where these ranges intersect the coast to produce dramatic, often vertical cliffs as at

Flamborough Head Flamborough Head () is a promontory, long on the Yorkshire coast of England, between the Filey and Bridlington bays of the North Sea. It is a chalk headland, with sheer white cliffs. The cliff top has two standing lighthouse towers, the olde ...

, the White Cliffs of Dover, Seven Sisters, Old Harry Rocks (Purbeck) and The Needles on the Isle of Wight. The Chalk, which once extended across the English Channel, gives rise to similar cliff features on the French coast.

Offshore and elsewhere

Northern Ireland

In the 'Ulster Cretaceous Province' of Northern Ireland the

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

-dominated Hibernian Greensands Group and the overlying

Ulster White Limestone Group The Ulster White Limestone Group is a late Cretaceous lithostratigraphic group (a sequence of rock strata) in Northern Ireland. The name is derived from the characteristic chalk rock which occurs particularly along the Antrim coast. The strata ...

are the stratigraphical equivalents of the Chalk Group of England. They are best exposed near the Antrim coast.

Scotland

In the '

Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European language family native to Scotland *Scottish English *Scottish national identity, the Scottish ide ...

Chalk Province' (extending from Mull to

Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated b ...

) the

Inner Hebrides Group In geology, the Inner Hebrides Group is a lithostratigraphical division containing a range of rocks mainly of Late Cretaceous age which occur around the west coast of the Scottish Highlands. It comprises the following formations: * Beinn Iadai ...

is the stratigraphical equivalent of England's Chalk Group. It comprises largely sandstones and

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

s though the

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. The ...

age Gribun Chalk Formation of Mull and nearby

Morvern Morvern, historically also spelt Morven, is a peninsula and traditional district in the Highlands, on the west coast of Scotland. It lies south of the districts of Ardgour and Sunart, and is bounded on the north by Loch Sunart and Glen Tarbert, ...

is recognised.

The Low Countries

The Dutch ( nl, Krijtkalk-Groep) and Belgian ( nl, Krijt-Groep) equivalents of the Chalk Group are basically continuous and crop out as a slightly northwest dipping

monocline A monocline (or, rarely, a monoform) is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence. Formation Monoclines may be formed in several different ways (see diagram) * ...

in a belt from the

German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...

city of

Aachen Aachen ( ; ; Aachen dialect: ''Oche'' ; French and traditional English: Aix-la-Chapelle; or ''Aquisgranum''; nl, Aken ; Polish: Akwizgran) is, with around 249,000 inhabitants, the 13th-largest city in North Rhine-Westphalia, and the 28th- ...

to the city of

Mons Mons (; German and nl, Bergen, ; Walloon and pcd, Mont) is a city and municipality of Wallonia, and the capital of the province of Hainaut, Belgium. Mons was made into a fortified city by Count Baldwin IV of Hainaut in the 12th century. T ...

, where they join Cretaceous deposits of the Paris Basin. North of

Namur Namur (; ; nl, Namen ; wa, Nameur) is a city and municipality in Wallonia, Belgium. It is both the capital of the province of Namur and of Wallonia, hosting the Parliament of Wallonia, the Government of Wallonia and its administration. Namu ...

the Cretaceous is overlain by younger Paleocene and Eocene deposits of the

Landen Group The Landen Group is a lithostratigraphic unit (a "group" of rock strata) in the Belgian subsurface. The group consists of two formations of Thanetian (late Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that ...

. In the Low Countries, the Chalk Group succession is divided into five formations, from top to base: *the

Houthem Formation The Houthem Formation, named after the Dutch town of Houthem, is a geological formation that crops out in the south of Belgian and Dutch Limburg. It has also been found in borings in the northeastern part of the Campine Basin. The formation co ...

, consisting of Paleocene calcareous arenites; *the

Maastricht Formation The Maastricht Formation (Dutch: ''Formatie van Maastricht''; abbreviation: MMa), named after the city of Maastricht in the Netherlands, is a geological formation in the Netherlands and Belgium whose strata date back to the Late Cretaceous, within ...

, consisting of Maastrichtian chalk and calcareous arenites; *the

Gulpen Formation Gulpen (; li, Gullepe ) is a village in the Dutch province of Limburg. It is approximately midway between the Dutch city of Maastricht and the German city of Aachen. Gulpen was a separate municipality until 1999, when it merged with Wittem. The ...

, consisting of Campanian to Maastrichtian chalk; *the Vaals Formation, consisting of alternating Campanian clays, glauconiferous sands and silts; *the

Aachen Formation The Aachen Formation ( nl, Formatie van Aken, german: Aachen-Formation) is an Upper Cretaceous geologic formation in the southern Netherlands and northeastern Belgium and adjacent Germany.Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. The ...

glauconiferous sands and silts. In Belgium, the Houthem Formation is sometimes not included in the Chalk Group because it is not a Cretaceous formation. Some stratigraphers therefore prefer to put it in the lower

Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...

Hesbaye Group The Hesbaye Group is a stratigraphic group (a set of sedimentary layers) in the subsurface of northeastern Belgium. This group was deposited during the early Paleocene epoch and is subdivided into two formations: the Houthem Formation (lower) and t ...

The English Channel

The

Channel Tunnel The Channel Tunnel (french: Tunnel sous la Manche), also known as the Chunnel, is a railway tunnel that connects Folkestone (Kent, England, UK) with Coquelles ( Hauts-de-France, France) beneath the English Channel at the Strait of Dover. ...

linking England and France was constructed by tunnelling through the West Melbury Marly Chalk (formerly the 'Chalk Marl' - a prominent sub-unit of the Grey Chalk Subgroup).

The North Sea

The chalk is also an important

petroleum reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence ...

in the North Sea Central

Graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

, mainly in Norwegian and

Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish ance ...

sectors and to a lesser extent in the United Kingdom Continental Shelf sector (UKCS). Across the north central and northern North Sea, the Chalk Group is a major seal unit, overlying a number of blocks of reservoir rocks and preventing their fluid contents from migrating upwards. North of the line of the Mid-North Sea - Ringkobing - Fyn structural high, the Chalk Group is still recognisable in drilled samples, but becomes increasingly muddy northwards. North of the Beryl Embayment (59°30' N 01°30'E), the Chalk Group is a series of slightly to moderately calcareous mudstones grouped under the name of the

Shetland Shetland, also called the Shetland Islands and formerly Zetland, is a subarctic archipelago in Scotland lying between Orkney, the Faroe Islands and Norway. It is the northernmost region of the United Kingdom. The islands lie about to the no ...

Group. With the exception of some thin sandy units in the inner

Moray Firth The Moray Firth (; Scottish Gaelic: ''An Cuan Moireach'', ''Linne Mhoireibh'' or ''Caolas Mhoireibh'') is a roughly triangular inlet (or firth) of the North Sea, north and east of Inverness, which is in the Highland council area of north of Scotl ...

, this sequence has neither source potential nor reservoir capacity and is not generally considered a drilling target. Its thickness and homogeneity does make it a common target for carrying out directional drilling manoeuvers. In the Shearwater and Eastern Trough Area Project areas (around 56°30' N 02°30'E, UKCS quadrants 22,23,29 and 30), the Chalk Group can be significantly overpressured. Further south in UKCS quadrant 30 and Norwegian quadrants 1 and 2, this overpressure helps preserve porosity and enables the Chalk to be an effective reservoir.

Reservoir stratigraphy

*Chalk Unit 6 -

Ekofisk Formation The Ekofisk Formation is a geological formation of Danian (lowermost Paleocene) age. It forms the uppermost part of the Chalk Group in the North Sea. It is an important reservoir for oil and gas in fields such as Ekofisk. It overlies the Maastrich ...

- Danian age (major reservoir in the

Ekofisk oil field Ekofisk is an oil field in block 2/4 of the Norwegian sector of the North Sea about southwest of Stavanger. Discovered in 1969 by Phillips Petroleum Company, it remains one of the most important oil fields in the North Sea. This was the first ...

and others) *Chalk Unit 5 -

Tor Formation The Tor Formation is a formation (geology), geological formation of late Campanian to Maastrichtian (uppermost Cretaceous) age. It forms the part of the Chalk Group in the North Sea. It is an important Petroleum reservoir, reservoir for oil and ga ...

- Maastrichtian age, (major reservoir in many fields including Joanne oil field (UKCS), Valhall oil field (NCS)) *Chalk Unit 4 - Hod Formation - Late Campanian age (mostly non reservoir) *Chalk Unit 3 - Hod Formation -

to Middle Campanian *Chalk Unit 2 - Hod Formation - Middle Turonian to

*Turonian shale - Early Turonian (non reservoir) *Plenus Marl - Late

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...

(non reservoir) *Chalk Unit 1 - Hidra Formation - Cenomanian (non reservoir)

Reservoir geology

The majority of Chalk reservoirs are redeposited

allochthon upright=1.6, Schematic overview of a thrust system. The hanging wall block is (when it has reasonable proportions) called a nappe. If an erosional hole is created in the nappe that is called a window (geology)">window. A klippe is a solitary out ...

ous beds. These include debris flows and turbidite flows. Porosities can be very high when preserved from diagenesis by early hydrocarbon charge. However, when these hydrocarbons are produced, diagenesis and compaction can restart which has led to several metres of subsidence at seabed, the collapse of a number of wells, and some ''extremely'' expensive remedial work to lift the platforms and re-position them.

Fossils

Fossils of the echinoid ''

'' from the Chalk Group have been studied for their continuous morphogical variation throughout the record. Mosasaur remains referred to "''Mosasaurus''" ''gracillis'' from the Turonian aged Chalk Group deposits actually are more closely allied to the Russellosaurina. A single partial maxillar tooth from Cenomanian aged Chalk Group described as "''Iguanodon hilli''" belongs to a non-Hadrosaurid

Hadrosauroid Hadrosauroidea is a clade or superfamily of ornithischian dinosaurs that includes the "duck-billed" dinosaurs, or hadrosaurids, and all dinosaurs more closely related to them than to ''Iguanodon''. Their remains have been recovered in Asia, Eu ...

.Dalla Vecchia FM. (2009b
European hadrosauroids
Actas de las IV Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno. Salas de los Infantes, Burgos, 45–74.